40    A MODIFIED CLAY AS ADSORBENT OF AN ORGANIC
POLLUTANT IN AQUEOUS AND MIXED SOLVENT
SYSTEMS
Valentine Nzengung, Graduate Research Assistant
Voudrais Evangelos, Assistant Professor
Department of Environmental Engineering
School of Civil Engineering
Marion Wampler, Associate Professor
Department of Geochemistry
School of Earth and Atmospheric Sciences
Georgia Institute of Technology
Atlanta, Georgia 30332
INTRODUCTION
Natural clays are ineffective adsorbents for poorly water-soluble organic contaminants such as
hydrophobic organic chemicals (HOCs), which are some of the pollutants transported from contaminated sites and petroleum spills into the subsurface environment and groundwaters.1-8 Clay minerals
possess a net negative electrical charge compensated for by inorganic exchange ions, (e.g., Na, and
Ca), which are strongly hydrated in the presence of water. This results in hydrophilic (water loving)
environment on the clay surface.
The surface properties of natural clays can be modified by simple ion-exchange reactions. Smectites
(Montmorillonite clays) are easily modified by exchanging inorganic cations with alkylammonium
cations also called quaternary ammonium cations (QUATS). By modifying clays in this manner the
inter-lamellar spacing is increased and the adsorptive surfaces of the clays are opened up to enhance
adsorption of organic molecules.1-4'9 More importantly, the substitution by organic cations reduces
the hydration of the clay and simultaneously decreases the "free" aluminosilicate mineral surface area
(i.e., the surface not covered by the organic exchange ions). As the inorganic ions are progressively
replaced by the organic cations, the surface properties of a clay containing mainly inorganic ions may
change considerably from highly hydrophilic to increasingly organophilic (hydrophobic).
The organic cations used commercially in the ion exchange process are almost exclusively alkylammonium salts (ALAS) with one or two long chain alkyl or aryl groups, i.e. they are cationic surfactants.10 Their classification as such is because in aqueous solution each molecule has a nonpolar
hydrocarbon group and a cationic, polar group. The molecular structures of these ALAS are diverse.
Depending on the hydrophobicity of the "tail", or R-group, the clays resulting from the exchange
reactions may be classified into two categories:2-4 (1) Organophilic clays produced from ALAS with
relatively long aliphatic R-groups. This may be represented by hexadecyltrimethylammonium
(HDTMA-), and Benzyldimethyltetradecylammonium (BDTDA-) smectites, where R is C16 and C,8
hydrocarbons. (2) Adsorptive clays, produced from ALAS with relatively short (less complex) Regroups.7'12 This may be represented by tetramethylammonium (TMA-) and trimethylphenylammo-
nium (TMPA-) smectites, where R is C4 and C9, respectively.
Adsorption studies from aqueous solution with natural smectite and organo-smectites as adsorbents
have been performed with a variety of HOCs. Boyd et al.3,4 and Jaynes and Boyd13 showed adsorption on the mineral surfaces was suppressed by water, and the uptake of nonionic organic chemicals
(NOCs) by HDTMA-smectite is essentially due to solute partitioning into the organic phase created
from the large C16 alkyl groups of HDTMA ions interacting with the clay. The organic matter derived
from exchanged HDTMA was 10 to 30 times more effective than natural soil organic matter in the
uptake (partition) of benzene, dichlorobenzene, and tetrachloroethylene from water.3,4
In many waste-disposal sites and in industrial wastewater treatment systems, there are solvents
aiding in mobilizing sparingly soluble organics. To analyze the sorption of HOCs from aqueous-
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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